Field of the Invention
This invention relates to a silencer for gas induction and exhaust systems. More particularly, the invention relates to an air induction system for an internal combustion engine comprising a novel silencer having a labyrinth configuration. The labyrinth configuration makes possible the packaging of an effective low frequency silencer in a limited space such as is characteristic of the engine compartment of an automobile.
A major source of noise from a gas induction or exhaust system of an internal combustion engine is the pulsating air flow the air intake valves in the cylinders resulting from the oscillatory motion of the pistons in the cylinders. The noise propagates in the flow duct which carries the air to the engine and can be characterized as a low frequency induction tone with a fundamental frequency f.sub.o which is proportional to the engine rpm. For a four-cycle engine, this frequency can be computed from EQU f.sub.o =CN/120
where C is the number of cylinders and N the rpm of the engine. For example, in a four cylinder engine the induction tone will have a frequency of 100 Hz at 3000 rpm. For an internal combustion engine, this frequency typically is less than 500 Hz, and in this low frequency range, the noise is transmitted through barriers and partitions, for instance into the passenger compartment in an automobile, with relatively little attenuation in comparison with noise at higher frequencies.
In view of the growing trend toward compact engine compartment design, the problem of effectively attenuating the low frequency (long wavelength) air induction noise has become increasingly more difficult because of space limitations. This is acknowledged in U.S. Pat. No. 4,800,985 which discloses the use of a side-branch tube which is either flexible or has a flexible portion. The tube is essentially a straight pipe configuration having a cross sectional area significantly smaller than that of the flow duct. The patentees refer to the tube as a high frequency attenuator implying that a tube long enough to cover the important low frequency end of the noise spectrum would exceed installation space limitations.
Another form of silencer is disclosed in U.S. Pat. No. 2,096,260. Damping tubes of considerable length closed at one end are positioned to cause sudden changes or reversals in direction of the path of the fluid flow within the damping tubes. These tubes are not relevant to the present invention since they are not resonator tubes but, to the contrary, are tubes which are designed to absorb the sound that enters into them, so that no resonance can occur. To achieve such absorption, the tubes are filled with porous material or provided with some other means of sound absorption.
There is a need for a silencer which is effective in attenuating low frequency noise and at the same time can be configured for placement in a confined space.